Eric Sallee is marketing manager for Sercel-GRC Corp. (www.sercel-grc.com), based in Tulsa, Okla. He is a graduate of the University of Oklahoma. Sallee can be reached at firstname.lastname@example.org.
Sucker rod pumping is the most common artificial lift method and is largely applied on marginal onshore oil fields. The sucker rod pumping system consists of a surface pumping unit that induces the alternate motion of a sucker rod string connected to a piston-type subsurface pump located at the bottom of the well. Higher profile wells completed with a sucker rod pump will use systems to automate the monitoring and control of the well and pump. However, these systems rely on algorithms instead of actual real-time downhole conditions.
Extending the run life of the pump while maximizing production is the goal of all sucker rod pump operators. The primary challenge is to do so without starving the pump and causing damage to expensive pump components. For years, operators have been searching for a reliable and accurate way to control sucker rod pumps and improve their production. Some ways this was accomplished were to employ a pump-off controller (POC) or dynamometer card or to implement an expensive pressure survey.
The best way for operators to make proactive decisions for the monitoring and control of a sucker rod pump is through the use of downhole sensors that provide continuous monitoring of bottom hole pressure and temperature. This is achieved by having the following:
• Having real-time well bore fluid level information
• Knowing the American Petroleum Institute (API) guidelines of the fluid produced, which can be easily converted by having a real-time bottom hole pressure and temperature readings at the surface
• Controlling the speed of the motor—using a proportional–integral–derivative (PID) loop—to control the rate at which the pump produces
Only with this information and control can operators maximize production and run life while minimizing costs.
Pump-off controllers (POCs) have been around for more than 30 years. More than 40,000 rod pumps are estimated to be equipped with a POC. POCs are proven to be useful in minimizing the wear and tear on rod pumping equipment while saving power. They operate by stopping the unit when the pump does not produce fluids. The major disadvantage to this is that the POC senses what is occurring downhole after an incident has already happened. Also, since it is primarily used as an on/off type control, the constant attempt to overcome the coefficient of static friction (constantly starting and stopping the rod string) causes extreme wear and tear to the rods and causes them to break more frequently. The rod string sees its highest stresses during this period.
More recently, POCs began to evolve into more than just a device to regulate the pump. These devices can now monitor motor speeds to avoid damage to the prime and gearbox. POCs can also sense rod loads that are out of tolerance and shut the system down the rods or the pump itself is damaged. A POC is also useful for monitoring torque and fluid levels. However, POCs are limited in increasing production because of the lack of real-time bottom hole pressure data.
The Importance of Pressure Knowledge
Understanding pressure and pressure relationships is important in well control. Specifically, bottom hole pressure data is of primary importance to understand the reservoir’s performance, well conditions and the sucker rod pumping system. The best way to know the correct pressure is by measuring the fluid level from the bottom of the well up. Using devices such as fluid shots and measuring fluid level from the top of the well down will sometimes give false readings due to emulsions of foam on top of the column of fluid. The only way to derive true bottom hole parameters is through the use of downhole sensors on the pump.
A client and major end user in the Middle East had been lifting its wells with sucker rod pumps. The POCs controlled the system and also for monitored parameters such as torque, rod load and fluid levels for these pumps. However, the client believed that he could gain more intelligence about the well and pump if more accurate pressure measurements were available and if it could confirm the algorithms from the POCs.
The client investigated conducting a pressure survey on his wells. However, the company quickly discovered that this was an expensive proposition. A typical pressure survey may cost between $20,000 and $30,000 in pulling hoist costs alone. This also becomes a reactive approach because conducting a pressure survey on a rod pump cannot be accomplished while the pump is operating. Because the bottom hole formation pressure is on a static well bore, a pressure survey would not provide the well operator with the data that access to the continuous real-time monitoring of pressure and temperature would provide. In addition, the access to this vital data can be achieved at the same price as just one workover.
Without downhole sensors, the end user did not know when to increase the pump speed and was unaware of any reduced pump efficiency. Alternatively, when the fluid level in the well bore was unknown, the end user would not know to reduce pump speed to prevent premature pump or rod failure. This critical information was unavailable using a POC alone, which provided only an estimate of the pump speed and control of the pump based upon the algorithm in the POC.
After more research, the company determined that deploying sensors downhole would be the best way to achieve maximum pump run-life and optimize production. The client selected a real-time, downhole monitoring system to be installed on its rod pumps. By combining the real-time, downhole sensor with the surface controller that was tied to the POC on the well, the client could adjust the pump speed according to the downhole pressure. This critical pressure data from the downhole monitoring system directed the pump to decrease production when the fluid level in the well bore decreased and increase production when the fluid level in the well bore increased.
This allowed the client to make day-to-day decisions on the pump speed, automatically and on a continuous basis, and provided a noticeable improvement in run life and production versus using a standalone POC or dynamometer card.
Whole Reservoir Monitoring
Downhole monitoring systems are indispensable tools when managing reservoir production, equipment run-life and performance by acting as an "eye into the reservoir." Producers using monitoring downhole have access to real-time well bore and pump information, which creates a competitive advantage in optimizing well bore and reservoir performance. This is important when attempting to optimize an entire field.
With all wells producing at a constant rate in a field, the fluid level should be relatively constant. If any well in the field has a change in its rate of production, all the wells will have a fluid level change. Without having a system that measures actual bottom hole pressure and automatically controls the pump production rate, the field will not produce at its optimum level. Seeing the entire reservoir through downhole monitoring allows a producer to measure the economical effectiveness of past well intervention decisions, leading to better future decisions. Most important, the well bore fluid level can be monitored proactively to make changes to the production rate before the fluid level drops beyond the point of no return and essentially runs the pump dry.
Recent estimates indicate that wells equipped with artificial lift pump monitoring systems can see a 10 to18 percent increase in production. To realize these increases in production, a reliable and accurate downhole sensor that measures pressure and temperature is required. An advanced surface data acquisition solution and qualified service personnel with field experience are also important to any successful artificial lift pump monitoring system.
The sensor is installed on a carrier sub in the tubing string above or below the pump. The sensor uses an electric instrument line to communicate data to the surface. At the core of the downhole sensor is patented capacitance transducer technology. This transducer is designed specifically for downhole monitoring of pumps and provides a rugged, reliable, value-added sensing solution that provides production managers with the ability to improve the decision-making process by giving them insight into the reservoir.
The surface data acquisition system—the datalogger—is packaged around a robust remote terminal unit (RTU) with multiple analog and digital input and output options. The data logger polls the downhole sensor and records and stores all the downhole pressure and temperature data. The data from the surface unit is transferrable to remote sites via integrated telecommunications systems.
The Critical Piece of the Puzzle
To have a fully automated closed-loop monitoring and control solution for an entire field of artificially lifted wells, the most important piece is knowing the flowing bottom hole pressure and fluid level. Since pressure and fluid level constantly change regardless of what is seen on the surface with monitoring motor speeds and production rates, optimized production is not possible without this downhole data.